U.S. patent number 8,647,737 [Application Number 13/250,926] was granted by the patent office on 2014-02-11 for method for fabrication of crack-free ceramic dielectric films.
This patent grant is currently assigned to UChicago Argonne, LLC. The grantee listed for this patent is Uthamalingam Balachandran, Sheng Chao, Shanshan Liu, Beihai Ma, Manoj Narayanan. Invention is credited to Uthamalingam Balachandran, Sheng Chao, Shanshan Liu, Beihai Ma, Manoj Narayanan.
United States Patent |
8,647,737 |
Ma , et al. |
February 11, 2014 |
Method for fabrication of crack-free ceramic dielectric films
Abstract
The invention provides a process for forming crack-free
dielectric films on a substrate. The process comprise the
application of a dielectric precursor layer of a thickness from
about 0.3 .mu.m to about 1.0 .mu.m to a substrate. The deposition
is followed by low temperature heat pretreatment, prepyrolysis,
pyrolysis and crystallization step for each layer. The deposition,
heat pretreatment, prepyrolysis, pyrolysis and crystallization are
repeated until the dielectric film forms an overall thickness of
from about 1.5 .mu.m to about 20.0 .mu.m and providing a final
crystallization treatment to form a thick dielectric film. Also
provided was a thick crack-free dielectric film on a substrate, the
dielectric forming a dense thick crack-free dielectric having an
overall dielectric thickness of from about 1.5 .mu.m to about 20.0
.mu.m.
Inventors: |
Ma; Beihai (Naperville, IL),
Balachandran; Uthamalingam (Willowbrook, IL), Chao;
Sheng (Greensburg, PA), Liu; Shanshan (Naperville,
IL), Narayanan; Manoj (Woodridge, IL) |
Applicant: |
Name |
City |
State |
Country |
Type |
Ma; Beihai
Balachandran; Uthamalingam
Chao; Sheng
Liu; Shanshan
Narayanan; Manoj |
Naperville
Willowbrook
Greensburg
Naperville
Woodridge |
IL
IL
PA
IL
IL |
US
US
US
US
US |
|
|
Assignee: |
UChicago Argonne, LLC (Chicago,
IL)
|
Family
ID: |
47992849 |
Appl.
No.: |
13/250,926 |
Filed: |
September 30, 2011 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20130084444 A1 |
Apr 4, 2013 |
|
Current U.S.
Class: |
428/216;
361/321.1; 427/379; 427/380; 427/372.2; 427/79 |
Current CPC
Class: |
H01L
21/02356 (20130101); H01L 21/02282 (20130101); H01L
28/55 (20130101); C23C 18/1254 (20130101); H01B
19/04 (20130101); H01L 21/3105 (20130101); C23C
18/1295 (20130101); H01L 21/02189 (20130101); C23C
18/1283 (20130101); H01L 21/02118 (20130101); H01L
21/02194 (20130101); C04B 35/624 (20130101); C23C
18/1225 (20130101); C23C 18/1241 (20130101); C23C
18/208 (20130101); C04B 35/4682 (20130101); C23C
18/1216 (20130101); C04B 35/493 (20130101); H01L
21/02186 (20130101); C04B 35/634 (20130101); H01L
21/02192 (20130101); H01L 21/02197 (20130101); C04B
2235/6562 (20130101); C04B 2235/3213 (20130101); C04B
2235/3227 (20130101); Y10T 428/24975 (20150115) |
Current International
Class: |
B32B
7/02 (20060101); B05D 5/12 (20060101); H01G
4/06 (20060101); B05D 3/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
U Balachandran, et al., Development of PLZT dielectrics on base
metal foils for embedded capacitors, J. Euro. Cerram. Soc. 30(2)
(2010), pp. 365-368. cited by applicant .
M. Narayanan, et al., Improved dielectric properties of lead
lanthanum zirconate titanate thin films on copper substrates,
Mater. Lett. 64 (1) (2010), pp. 22-24. cited by applicant .
H. Kozuka, et al., Single-Step Deposition of Gel-Derived Lead
Zirconate Titanate Films Critical Thickness and Gel Film to Ceramic
Film Conversion, Am. Ceram. Soc., 85 (11) (2002), pp. 2696-2702.
cited by applicant .
H. Kozuka, et al., Single-Step Dip Coating of Crack-Free BaTi3
Films >1 micron Thick: Effect of Poly (vinypyrrolidone) on
Critical Thickness, J. Am. Ceram. Soc. 83(5), (2000), pp.
1056-1062. cited by applicant .
A. Yamano, et al., Effects of the heat-treatment conditions on the
crystalographic orientation of Pb(Zr1Ti)O3 Thin films Prepared by
polyvinylpyrrolidone-assisted sol-gel method, J. M. Ceram. Soc. 90
(12) (2007), pp. 3882-3889. cited by applicant .
Z. H. Du, et al., "Densification of the PLZT Films Derived from
Polymer-Modified Solution for Tailoring Annealing Conditions," J.
Am. Ceramic Society 90 (3), (2007), pp. 815-820. cited by applicant
.
B. Ma, et al., Chemical solution deposition of ferroelectric lead
lanthanum zirconate titanate films on base-metal foils J
Electroceram (2009), pp. 383-389. cited by applicant .
Y. Liu, et al., Nucleation - or Growth-Controlled Orientation
Development in Chemically Derived Ferroelectric Lead Zirconate
Titanate (Pb(Zrx Ti1-x) O3, x=0.4) Thin Films, J Am Ceram. Sox.
79(2) (1996), pp. 495-498. cited by applicant .
Z. H. Du, et al., Effect of polyvinylpyrrolidone on the formation
of perovskite phase and rosette-like structure in sol-gel-derived
PLZT films, J. Mater. Res., vol. 22 (8) (2007), pp. 2195-2203.
cited by applicant .
B. Ma, et al., Fabrication and dielectric property of ferroelectric
PLZT films grown on metal foils, Materials Research Bulletin 46(7)
(2011), pp. 1124-1129. cited by applicant .
B. Ma, et al., Dielectric Properties of PLZT film-on-foil
capacitors, J.Materials Letters 62 (2008), pp. 3573-3575. cited by
applicant .
S. Chao et al., Effects of sintering temperature on the
microstructure and dielectric properties of titanium dioxide
ceramics, J Mater Sci 45 (2010), pp. 6685-6693. cited by applicant
.
B. Ma, et al., Dielectric strength and reliability of ferroelectric
PLZT films deposited on nickel substrates, Materials Letters 63
(2009), pp. 1353-1356. cited by applicant.
|
Primary Examiner: Ewald; Maria Veronica
Assistant Examiner: Gugliotta; Nicole T
Attorney, Agent or Firm: Cherskov Flaynik & Gurda
LLC
Government Interests
CONTRACTUAL ORIGIN OF THE INVENTION
The U.S. Government has rights in this invention pursuant to
Contract No. DE-AC02-06CH11357 between the U.S. Department of
Energy and the University of Chicago representing Argonne National
Laboratory.
Claims
The invention claimed is:
1. A crack-free dielectric comprising: a substrate; a dielectric
layer overlaying the substrate, whereby the layer is formed by
depositing a first dielectric precursor sol-gel layer having a
thickness from about 0.3 .mu.m to about 1.0 .mu.m on a substrate,
heating the first dielectric precursor sol-gel layer at a low
temperature from about 100.degree. C. to about 200.degree. C. for a
low temperature heating time from about 1 minute to about 30
minutes, increasing the temperature from about 275.degree. C. to
about 325.degree. C. in a prepyrolysis step and maintaining the
temperature at the prepyrolysis temperature for a prepyrolysis
period of time, increasing the temperature from about 375.degree.
C. to about 425.degree. C. in a first pyrolysis step and
maintaining the temperature at the first pyrolysis temperature for
a first pyrolysis period of time, increasing the temperature from
about 425.degree. C. to about 475.degree. C. in a second pyrolysis
step and maintaining the temperature at the second pyrolysis
temperature for a second pyrolysis period of time, increasing the
temperature from about 600.degree. C. to about 800.degree. C. for a
period of time to crystallize at least one layer; repeating the
deposition step, initial heating step, pyrolyzing and
crystallization steps to form a dielectric precursor layer having a
total thickness from about 1.5 .mu.m to about 20.0 .mu.m on the
substrate to form a crystallized dielectric layer.
2. The crack-free dielectric of claim 1 further comprising heating
the substrate and the dielectric precursor to crystallize the
dielectric precursor at a temperature from about 625.degree. C. to
about 675.degree. C. for a final crystallization time to form
crystallized dielectric layer.
3. The crack-free dielectric of claim 1 wherein the dielectric
precursor is a titanate selected from the group consisting of
lanthanum doped lead zirconate titanate soluble gel solution (PLZT
sol-gel) containing polyvinylpyrrolidone and a barium strontium
titanate (BST) soluble gel solution sol-gel containing
polyvinylpyrrolidone.
4. The crack-free dielectric of claim 1 wherein the low temperature
heating time is from about 2 minutes to about 10 minutes.
5. The crack-free dielectric of claim 1 wherein the deposition
step, initial heating step, pyrolyzing and crystallization steps
are repeated to form a dielectric precursor layer having a
thickness from about 2.0 .mu.m to about 5.0 .mu.m on a
substrate.
6. The crack-free dielectric of claim 2 wherein the final
crystallization time is from about 10 to about 40 minutes.
7. The crack-free dielectric of claim 1 wherein the substrate is
selected from the group consisting of silicon, a platinized silicon
wafer and a base metal.
Description
FIELD OF THE INVENTION
The present invention relates to formation of thick ceramic
dielectric films. More specifically this invention relates to the
formation of thick crack-free ceramic dielectric films, such as
lanthanum doped lead zirconate titanate (PLZT) or barium strontium
titanate (BST), having an overall thickness of from 1.5 to about 20
.mu.m.
BACKGROUND OF THE INVENTION
The recent need for passive power electronics with improved
performance, high reliability, and reduced size and weight, has
driven interest in ceramic film on metallic substrates in these
applications. The ceramic films on metal foils, known as
"film-on-foil" technology, in which ceramic films were deposited on
base metal foils for embedding into a printed circuit board. The
interest in "film-on-foil" technology exploits ceramic dielectrics,
including the important properties, such as ferroelectric,
piezoelectric, pyroelectric and electro-optic properties. These
properties are utilized in manufacture of nonvolatile semiconductor
memories, thin-film capacitors, pyroelectric infrared (IR)
detectors, sensors, surface acoustic wave substrates, optical
waveguides, and optical memories. Recently, there has been
increased interest in applying "film-on-foil" to power electronics,
such as capacitors with high capacitance required to work at high
voltages. Applying the film-on-foil technology can substantially
reduce the production cost and improve the volumetric and
gravimetric efficiencies of the capacitors.
Important ferroelectric materials for thin-film applications are
typically titanates and niobates with oxygen-octahedral structure
types, such as the perovskite structure. Examples of such
ferroelectric perovskites include lead titanate (PbTiO.sub.3), lead
zirconate (PbZrO.sub.3), lead zirconate titanate [Pb(Zr,Ti)O.sub.3
or PZT], lead lanthanum titanate [(Pb,La)TiO.sub.3], lead lanthanum
zirconate [(Pb,La)ZrO.sub.3], lead lanthanum zirconate titanate
[(Pb,La)(Zr,Ti)O.sub.3 or PLZT], lead magnesium niobate
[Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3], lead zinc niobate
[Pb(Zn.sub.1/3Nb.sub.2/3)O.sub.3], strontium titanate
(SrTiO.sub.3), barium titanate (BaTiO.sub.3), barium strontium
titanate [(Ba,Sr)TiO.sub.3], barium titanate zirconate
[Ba(Ti,Zr)O.sub.3], potassium niobate (KNbO.sub.3), potassium
tantalate (KTaO.sub.3), and potassium tantalate niobate
[K(Ta,Nb)O.sub.3]. Device applications of ferroelectric thin films
require that bulk ferroelectric properties be achieved in thin
films. The physical and chemical properties of the film (density,
uniformity, stoichiometry, crystal structure, and microstructure)
are extremely important. The utilization of ferroelectric thin
films for electronic and optical applications has been hindered by
the lack of production processes to form deposits of sufficient
thickness.
Particular interest has shown that lead lanthanum zirconate
titanate (Pb.sub.0.92La.sub.0.08Zr.sub.0.52Ti.sub.0.48O.sub.3,
PLZT) films deposited on nickel or copper foils possessed excellent
dielectric properties, which are promising for high power
applications such as plug-in hybrid electric vehicles. In power
electronics, capacitors with high capacitance are required to work
at high voltages, typically in the range of 450 to 600 V. This
requirement imposes an additional challenge to fabricate thicker
films (>1 .mu.m) that can withstand high voltage. However, in
the fabrication process, the deposited films crack easily during
heat treatment, due to the well-known critical thickness effect.
Due to this effect per-layer thickness that can be achieved by
conventional sol-gel methods is generally limited to about 0.2
.mu.m, thus making these methods unattractive to industry if
thicker films are needed.
It has been reported that the critical thickness of lead zirconate
titanate (PZT) films can be substantially increased by introducing
polyvinylpyrrolidone (PVP) into sol-gel solutions (H. Kozuka and S.
Takenaka, J. Am. Ceram. Soc. 85 (11) (2002) 2696-2702) and barium
titanate (H. Kozuka and M. Kajimura, J. Am. Ceram. Soc. 83 (5)
(2000)1056-1062). The increased critical thickness of the PZT
dielectric is attributed to the structural relaxation effect as PVP
suppressed the condensation reaction because of the strong hydrogen
bonds between the amide groups of PVP and the hydroxyl groups of
the metalloxane polymers (H. Kozuka and M. Kajimura, J. Am. Ceram.
Soc. 83 (5) (2000)1056-1062). However, thick films derived from
PVP-containing solutions were generally found to be porous due to
the thermal decomposition of PVP during heating (A. Yamano and H.
Kozuka, J. Am. Ceram. Soc. 90 (12) (2007)). Pyrolysis temperature
had been shown to have a significant impact on microstructure of
the films derived from PVP-modified sol-gel process (Z. H. Du, J.
Ma, and T. S. Zhang, J. Am. Ceram. Soc. 90 (3) (2007)).
The efficient removal of decomposition byproducts produced by
processing aids during dielectric fabrication and the consolidation
of the film raises significant processing issues.
SUMMARY OF INVENTION
An object of the invention is to provide a thick dielectric film
having and a method for forming overcomes many of the disadvantages
of the prior art films.
Another object of the present invention is to provide a process for
the manufacture of a crack-free ceramic film having an
overall/final thickness from 1.5 .mu.m to about 20.0 .mu.m. In an
embodiment the overall thickness is from about 2.0 .mu.m to 10.0
.mu.m. In another embodiment the overall thickness is from about
2.0 .mu.m to 5.0 .mu.m. A feature of the invention is to provide a
heat treatment process that reduces stresses brought about
differences in thermal expansion in the formation of thick
dielectric films. An advantage of the invention is to permit the
placement of multiple layers to form a dielectric with a
substantial thickness without formation of thermal stress
cracks.
Another object of the present invention is to provide a process for
the manufacture dense dielectric ceramic film. A feature of the
invention is a process that permits coalescence of a dielectric and
the reduction of void space. An advantage of the invention is the
formation of a ceramic having reduced voids thereby forming a
continuous dense material.
Another object of the present invention is to provide a process,
the method for forming a dielectric ceramic having a higher
dielectric constant compared to materials fabricated by typically
processing methods. A feature of the invention is to provide a
heating process that efficiently removes most of the processing
component that would normally lower the dielectric constant of the
final film. An advantage of the invention is the formation of dense
ceramic containing fewer forming aids that lower the dielectric
constant of the dielectric film.
In brief, the invention provides a process for forming crack-free
dielectric films on a substrate, the process comprising the
application of a dielectric precursor layer to a substrate, low
temperature heat pretreatment, staged prepyrolysis, pyrolysis and
crystallization step for each layer, repeated until the dielectric
film forms a total or overall thickness of from about 1.5 .mu.m to
about 20.0 .mu.m and providing a final crystallization treatment to
form a thick dielectric film. In an embodiment of the invention,
the total thickness is from about 2.0 .mu.m to about 10.0 .mu.m on
a substrate (preferably 2.0 .mu.m to about 5.0 .mu.m). Also
provided was a thick crack-free dielectric film on a substrate, the
dielectric forming a dense thick crack-free dielectric having an
overall dielectric thickness of from about 1.5 .mu.m to about 20.0
.mu.m.
The invention provides a process for forming a crack-free
dielectric, the process includes providing a substrate and
providing a dielectric precursor soluble gel solution. An initial
dielectric precursor sol-gel layer having a thickness from about
0.3 .mu.m to about 1.0 .mu.m is deposited on a substrate. The first
dielectric precursor sol-gel layer is heated at a low temperature
preheat from about 100.degree. C. to about 200.degree. C. for a
period of from about 1 minute to about 30 minutes. In an embodiment
of the invention temperature preheat from about 100.degree. C. to
about 180.degree. C. The temperature is increased to a prepyrolysis
(or low temperature pyrolysis) at a temperature from about
275.degree. C. to about 325.degree. C. in a prepyrolysis step and
maintained at the prepyrolysis temperature for a prepyrolysis
period of time. In an embodiment, the prepyrolysis temperature is
from about 285.degree. C. to about 315.degree. C. The temperature
is then increased to a first pyrolysis temperature from about
375.degree. C. to about 425.degree. C. and maintained for a first
pyrolysis period of time. The temperature is then increased to a
second pyrolysis temperature from about 425.degree. C. to about
475.degree. C. in a second pyrolysis step and maintained at the
temperature at the second pyrolysis temperature for a second
pyrolysis period of time. The temperature is then raised to a
crystallization temperature of from about 600.degree. C. to about
800.degree. C. for a period of time to crystallize at least one
layer. The deposition step, initial heating step, pyrolyzing steps
and crystallization step are repeated to form a dielectric
precursor layer having a total thickness from about 1.5 .mu.m to
about 20.0 .mu.m on a substrate. In an embodiment of the invention,
the total thickness of the dielectric film is 2.0 .mu.m to about
10.0 .mu.m on a substrate. In one embodiment the total thickness
from about 2.0 .mu.m to about 5.0 .mu.m. In one embodiment of the
invention a final densification heating step wherein the substrate
and the dielectric precursor are heated to crystallize the
dielectric precursor at a temperature from about 600.degree. C. to
about 800.degree. C. for a final crystallization of time to form
crystallized dielectric layer. In another embodiment of the
invention, a final densification heating step wherein the substrate
and the dielectric precursor are heated to crystallize the
dielectric precursor at a temperature from about 650.degree. C. to
about 750.degree. C. for a final crystallization of time to form
crystallized dielectric layer. Preferably, the process forms a
dielectric film layer having a total thickness from about 1.5 .mu.m
to about 20 .mu.m on a substrate. In one embodiment of the
invention the process forms a dielectric layer having a total
thickness from about 2.0 .mu.m to about 10 .mu.m (preferably from
about 2.0 .mu.m to about 5.0 .mu.m). The step-wise preheat
treatment (SPT) produces a dielectric film producing superior
electrical properties over dielectric films produce by the
conventional rapid thermal annealing (RTA) process. In the RTA
process, films placed in an alumina boat were directly inserted
into a tube furnace preheated at 450.degree. C. In the SPT process,
films were preheated at 300.degree. C. for 5 min, then 400.degree.
C. for another 5 min, and finally at 450.degree. C. for 10 min (by
moving the film into different hot zones with designated
temperatures in an electric furnace).
In an embodiment of the invention, the low temperature heating time
is from about 2 minutes to about 10 minutes. In another embodiment
of the invention, the prepyrolysis, first and second pyrolysis
times are from about 4 to about 10 minutes. Still, in another
embodiment of the invention, the final crystallization time for the
process is from about 5 to about 40 minutes. In another embodiment
of the invention, the final crystallization time is from about 10
minutes to about 20 minutes. The process can be used to for a
dielectric layer from any dielectric film forming material;
preferably selected from lanthanum doped lead zirconate titanate
soluble gel solution (PLZT sol-gel) or a barium strontium titanate
(BST). The substrate can be any suitable substrate, such as a
silicon wafer, a platinized silicon wafer, a base metal (nickel,
copper, iron or chromium) or an alloy of a base metal (Hastelloy C
or Inconel 625).
BRIEF DESCRIPTION OF DRAWING
The invention together with the above and other objects and
advantages will be best understood from the following detailed
description of the preferred embodiment of the invention shown in
the accompanying drawings, wherein:
FIG. 1 is an XRD profile of the PLZT films prepared by the SPT and
RTA with different PLZT: PVP ratios.
FIG. 2. SEM images showing the surface and cross-sectional (insets)
morphology of the PLZT films: (a) RTA, PLZT:PVP=1:2; (b) SPT,
PLZT:PVP=1:2; (c) RTA, PLZT:PVP=1:3; and (d) SPT, PLZT:PVP=1:3.
FIG. 3. Dielectric constant (k) and dissipation factor (D.F.) of
the PLZT films as a function of bias field: (a) PLZT:PVP=1:2,
20.degree. C.; (b) PLZT:PVP=1:2, 150.degree. C.; (c) PLZT:PVP=1:3,
20.degree. C.; and (d) PLZT:PVP=1:3, 150.degree. C.
FIG. 4. Hysteresis loops of the PLZT films at room temperature: (a)
PLZT:PVP=1:2 and (b) PLZT:PVP=1:3.
FIG. 5. Current density relaxation of the PLZT films measured at
100 kV/cm at room temperature. Symbols are experimental data and
solid lines are the fitting curves.
FIG. 6. Weibull plots and corresponding parameters for dielectric
breakdown strength of the films derived from the PLZT:PVP=1:2
solution and pyrolyzed by RTA and SPT.
DETAILED DESCRIPTION OF THE INVENTION
The foregoing summary, as well as the following detailed
description of certain embodiments of the present invention, will
be better understood when read in conjunction with the appended
drawings.
As used herein, an element or step recited in the singular and
preceded with the word "a" or "an" should be understood as not
excluding plural said elements or steps, unless such exclusion is
explicitly stated. Furthermore, references to "one embodiment" of
the present invention are not intended to be interpreted as
excluding the existence of additional embodiments that also
incorporate the recited features. Moreover, unless explicitly
stated to the contrary, embodiments "comprising" or "having" an
element or a plurality of elements having a particular property may
include additional such elements not having that property.
Dielectric films are used in electrical components, particularly
capacitors, to inhibit/control the flow of electricity for power
management applications. In power electronics applications, such as
electric vehicles, capacitors with high capacitance are required to
operate at voltages in excess of 100 volts (V). In one embodiment
of the invention, capacitors formed from the process of the
invention operate at voltages in excess of 50 volts. Typically,
this would require a material with a dielectric constant
(.kappa.).apprxeq.1000 at zero bias. This requirement imposes an
additional challenge to fabricate thicker films in the range of 1.5
.mu.m to 20 .mu.m, while providing a film substantially free from
cracks and voids. To achieve this objective the film must form a
dense layer substantially free from defects and cracks. The use of
processing additives, such as polyvinylpyrrolidone (PVP)--increase
the viscosity of the coating solution. The use of the viscosity
modifiers raises additional processing issues in the efficient
removal of processing aids and modifiers to form a dense dielectric
film on a substrate. To realize this goal the inventors have
developed a dielectric/support curing process for the fabrication
of dense crack-free dielectrics while reducing the overall
fabrication time.
The inventors have developed a dielectric fabrication process for
the formation of a thick dense dielectric substantially free from
voids and cracks that reduce the dielectric constant of the film.
Further the invented process reduces cracking of the dense film
thereby significantly reducing the leakage current density
(A/cm.sup.2) and improving the dielectric breakdown strength of the
film. The invented process effectively fuses/melds multiple layers,
thereby consolidating the dense film into one continuous film,
while significantly reducing cracks/fractures.
The invention provides a process for forming crack-free dielectric
films on a substrate. A dielectric precursor layer is applied to a
substrate. Initially, the precursor is heated in low temperature
heat pretreatment step, followed by prepyrolysis, pyrolysis (in two
stages) and crystallization step for each layer. The, steps of
deposition, low temperature heat pretreatment, staged prepyrolysis,
pyrolysis and crystallization are repeated until the dielectric
film forms an overall thickness of from about 1.5 .mu.m to about
20.0 .mu.m and providing a final crystallization treatment to form
a thick dielectric film. In an embodiment of the invention the
total thickness is from about 2.0 .mu.m to about 10 .mu.m. Also
provided was a thick crack-free dielectric film on a substrate, the
dielectric forming a dense thick crack-free dielectric having an
overall dielectric thickness of from about 1.5 .mu.m to about 20.0
.mu.m. Surprisingly the inventors have discovered that the invented
process produces a dense film having substantially fewer racks and
voids. The inventors have discovered that the invented process
increases the dielectric constant of the film by approximately 50%
above the dielectric constant produced from the process of the
invention. This surprising increase in the dielectric constant
combined with a reduced dissipation factor (DF) for dielectric
films produced by the invention provides a dielectric film
providing increase performance.
Experimental Procedure
PLZT precursor solutions of 0.6 M concentration were prepared by a
modified 2-methoxyethanol synthesis route by using following raw
materials: 99% lead acetate tri-hydrate, 97% titanium isopropoxide,
70% zirconium n-propoxide in 1-propanol, and 99.9% lanthanum
acetate hydrate (all from Sigma-Aldrich Co.). The solution contains
20% excess lead to compensate for the loss during treatment. To
form the chemical solution for deposition, PVP (PVP10,
Sigma-Aldrich Co., with an average molecular weight of 10,000
g/mol) was added to the PLZT stock solution in PLZT:PVP=1:2 and 1:3
molar ratio (PVP is defined by its monomer). The PVP-added PLZT
solution was aged for approximately 12 h before coating. The aged
solution, after passing through a 0.2-.mu.m syringe filter, was
deposited on a substrate by means of a spin coater (Laurell
Technologies, North Wales, Pa.) at 2000 rpm for 30 sec. The
substrate was a platinized silicon wafer with .apprxeq.100-nm thick
Pt layer (Nova Electronic Materials, Flower Mound, Tex.). After
deposition the films were first preheated at 100.degree. C. to
200.degree. C. for 1 to 30 min in a furnace. Then, the films were
subjected to two different pyrolysis processes: rapid thermal
annealing (RTA) and step-wise preheat treatment (SPT). In the RTA
process, films placed in an alumina boat were directly inserted
into a tube furnace preheated at 450.degree. C. for 10 min. In the
SPT process, films were preheated at 300.degree. C. for 5 min, then
400.degree. C. for another 5 min, and finally at 450.degree. C. for
10 min (by moving the film into different hot zones with designated
temperatures in an electric furnace). After pyrolysis, all films
were crystallized at 650.degree. C. for 10 min. The deposition,
pyrolysis, and crystallization steps were repeated to build up a
thickness to about 1.6 .mu.m for all samples; in order to avoid the
possible thickness effect on dielectric properties. Final
crystallization and densification were conducted at 650.degree. C.
for 30 min. All heat treatments were performed in ambient
atmosphere.
Platinum (100-nm thickness) was deposited on samples through a
shadow mask by electron-beam evaporation as top electrodes. Samples
with Pt top electrodes were annealed at 450.degree. C. in air for 5
min for electrode conditioning. A Signatone QuieTemp.RTM. probe
system with heatable vacuum chuck (Lucas Signatone Corp., Gilroy,
Calif.) was used for dielectric property characterization. Phase
identification was conducted by using a Bruker D8 AXS
diffractometer with General Area Detector Diffraction System.
Microstructure observation was performed with a Hitachi S4700
field-emission scanning electron microscope (SEM). An Agilent
E4980A Precision LCR Meter was used to measure capacitance and
dissipation factor at an applied bias field. Then, dielectric
constant was calculated with the diameter of the electrode
(250-.mu.m diameter electrode was used for all electrical tests)
and the thickness of the film. Hysteresis loops were measured by
using a Radiant Technologies Precision Premier II tester using a
field sweeping frequency of 1 kHz. Breakdown strength and
current-voltage characteristics were measured by using a Keithley
237 high-voltage source meter.
FIG. 1 shows XRD patterns of the PLZT films prepared by SPT- and
RTA-process with PLZT:PVP ratios of 1:2 and 1:3. For comparison,
XRD pattern of a PLZT sample prepared with the same PLZT stock
solution but without PVP is also included. (111) preferential
orientation was observed for this sample; however, all the other
samples with PVP showed a random orientation, with (110) as the
most intense peak. The (111) preferential orientation was
attributed to the lattice matching between the (111)-oriented Pt
substrate and the PLZT film, while the absence of this orientation
is likely related to the PVP decomposition. Yamano and Kozuka
believed that orientation became difficult in PZT films derived
from the solution containing PVP due to the large number of
nucleation sites provided by the porosity. Except for this
difference, all samples are phase-pure perovskite without any
detectable secondary phases, such as pyrochlore.
Film thickness was determined from the SEM cross-sectional images
(FIG. 2). The thicknesses of the PLZT films after 5 or 6 deposition
cycles were in the range of 1.5-1.7 .mu.m, corresponding to
per-layer thickness of .apprxeq.266 nm and .apprxeq.340 nm for
solutions with PLZT:PVP=1:2 and 1:3 ratio, respectively. The SEM
cross-sectional image for the RTA process is shown in FIG. 2(a)
(1:2 ratio) and 2(c) (1:3 ratio). The SEM cross-sectional image for
the inventors' SPT process is shown in FIG. 2(b) (1:2 ratio) and
2(d) (1:3 ratio). The film thickness was not influenced by the
pyrolysis methods. The relatively low per-layer thickness is due to
the low molecular weight PVP used in the present study, as layer
thickness is mainly determined by viscosity of the solution. The
inventors did not observe the so-called "rosette" structure, which
is common in Pb-containing ferroelectric films derived from polymer
[PVP or poly(ethylene glycol)]-modified sol-gel solutions (Z. H. Du
and T. S. Zhang, and J. Ma, J. Mater. Res. 22 (8) (2007)
2195-2203).
The inventors observed two trends in the examination of the surface
morphology of the films. First, the number of the pores and their
size increased with increasing PVP addition. Second, for samples
prepared from the same solution (same PVP content), pore sizes were
smaller in the SPT-treated samples. The more PVP added, the more
polymer was burned out eventually; therefore, it is reasonable to
expect more residual pores left in the films prepared from the
solution with high PVP content. The inventors observe that,
although the final pyrolysis temperature was the same (450.degree.
C.), films treated with additional thermal heating steps at lower
temperatures demonstrated a higher degree of integrity. Previous
research showed that PVP starts to decompose to carbonaceous
species in the temperature range of 250-320.degree. C., and the
carbonaceous species are oxidized at .apprxeq.360-460.degree. C.
(H. Kozuka and S. Takenaka, J. Am. Ceram. Soc. 85 (11) (2002)
2696-2702). Therefore, we hypothesized that additional heating
stages at 300.degree. C. and 400.degree. C. would assist PVP to
decompose in a gradual manner, preventing it from directly
decomposing into gaseous species in a violent manner, which likely
causes the formation of large pores and even cracks.
FIG. 3 plots dielectric constant and dissipation factor as a
function of applied bias field for the SPT- or RTA-treated samples
with PLZT:PVP=1:2 and 1:3; The properties were measured at
20.degree. C. and 150.degree. C. The SPT-treated samples have much
higher dielectric constant than the RTA-treated samples processed
under similar conditions. The inventors observed the dielectric
constant for the SPT-treated vs. the RTA-treated samples increases
by .apprxeq.38% and .apprxeq.50% at 20.degree. C. (FIG. 3a) and
150.degree. C., (FIG. 3b) respectively, for the PLZT:PVP=1:2
solution, and it increases by .apprxeq.44% and .apprxeq.56% at
20.degree. C. (FIG. 3c) and 150.degree. C. (FIG. 3d), respectively,
for the PLZT:PVP=1:3 solution. For samples derived from the
solutions with a given PLZT to PVP ratio, since XRD analysis did
not reveal any preferred crystallographic orientation and secondary
phase. The inventors believe that the difference in dielectric
constant is attributed to the difference in microstructures as a
result of the different pyrolysis conditions. Furthermore,
dielectric constant for both the SPT- and the RTA-treated samples
drops rapidly with the increase of the PVP content in the precursor
solution. As shown in FIG. 2, samples with various PVP contents and
pyrolyzed in different ways exhibited different microstructures. In
general, SPT-treated samples with less PVP content (FIGS. 2a and
2b) exhibited a denser microstructure, and these samples thus
showed higher dielectric constant. The dielectric constant values
of the RTA-treated samples with PLZT:PVP=1:2 are close to those of
PLZT films deposited by an acetic-acid-based sol-gel process
without PVP addition [1]. Dissipation factor of these samples is
about 5-6% at room temperature, and it decreased slightly to 4-5%
at 150.degree. C., which is also at the same level as our previous
results.
FIG. 4 shows polarization-electric field (P-E) hysteresis loops of
the PLZT films measured at room temperature. A high electric field
up to 1000 kV/cm was applied. In general, all films show a
relatively slim hysteresis loop, which is desirable for energy
storage applications, as the area enclosed by the charge and
discharge curves represents energy loss. Note that with increasing
of PVP content, the shape of the loop starts to become "fatty," it
means that a larger portion of the electric energy stored would not
be retrievable upon discharge. In addition, we found an increase in
average remnant polarization [P.sub.r=(+P.sub.r-(-P.sub.r))/2] and
a decrease in average coercive field
[E.sub.c=(+E.sub.c-(-E.sub.c))/2] when the pyrolysis process was
changed from RTA to SPT for the films derived from the solution
with same PLZT:PVP ratio. For the PLZT:PVP=1:2 solution, P.sub.r
increased from 13.3 to 13.9 .mu.C/cm.sup.2, while for the
PLZT:PVP=1:3 solution, P.sub.r increased from 12.4 to 14.5
.mu.C/cm.sup.2. In terms of coercive field, E.sub.c decreased from
83 to 59 kV/cm for the PLZT:PVP=1:2 and from 112 to 91 kV/cm for
the PLZT:PVP=1:3 solution. Again, these differences can be
explained by the microstructural features.
Time-relaxation data of leakage current density at 100 kV/cm are
given in FIG. 5. The decay in dielectric relaxation current obeys
Curie-von Schweidler equation (Jonscher, Dielectric Relaxation in
Solids, Chelsea Dielectrics Press (1983)) J=J.sub.s+J.sub.0t.sup.-n
(1) where J.sub.s is the steady-state current density, J.sub.0 is a
fitting constant, t is the relaxation time in second, and n is the
slope of the log-log plot. The calculated steady-state current
densities are listed in table 1.
TABLE-US-00001 TABLE 1 Steady-state leakage current densities of
the PLZT films. Leakage current Samples density (A/cm.sup.2) 1:2
RTA 1.38 .times. 10.sup.-8 1:2 SPT 5.43 .times. 10.sup.-9 1:3 RTA
7.20 .times. 10.sup.-8 1:3 SPT 4.12 .times. 10.sup.-8
We can see that it follows a logical trend that films with high PVP
content and pyrolyzed with the RTA process show higher leakage
current. In addition to the microstructural defects that can be
used to interoperate this difference, more residual carbon left
inside the films with high PVP content and pyrolyzed in a rapid way
may also contribute to high leakage current. The leakage current
value measured for the SPT-treated sample with PLTZ:PVP=1:2 is
close to that of the films deposited on nickel substrates using the
same chemical solution but without PVP (B. Ma, D. K. Kwon, M.
Narayanan, and U. Balachandran, Mater. Lett. 62 (2008)
3573-3575).
Dielectric breakdown strength (BDS) was measured on 25 samples
tested in a top-to-bottom electrode configuration. Failure of the
sample was defined by a 1-.mu.A criterion. The BDS data for the
films derived from the PLZT:PVP=1:2 solution presented as Weibull
plots (B. Ma, D. K. Kwon, M. Narayanan, and U. Balachandran, Mater.
Lett. 62 (2008) 3573-3575), (FIG. 6) due to the inherently
statistical nature of failure. The samples treated by the SPT
process show slightly higher mean BDS (BDS.apprxeq.2.1 MV/cm) than
the samples treated by the RTA process (BDS.apprxeq.1.9 MV/cm).
Furthermore, their Weibull moduli (.beta.) exhibit a larger
difference (.beta.=9.5 for the RTA sample and .beta.=27.3 for the
SPT sample). Weibull modulus is a measure of distribution of the
data: the higher the value of .beta., the smaller the variation of
the data. Thus, a higher modulus indicates a better representation
of the sample-to-sample performance as measured by mean breakdown
strength. Here, the higher modulus found in the SPT-treated samples
is attributed to their denser microstructure (FIG. 2), as residual
porosity (especially so-called "critical flaws" that initiate the
breakdown process) is always detrimental to the breakdown strength
(S. Chao, V. Petrovsky, and F. Dogan, J. Mater. Sci. 45 (2010)
6685-6693). High breakdown strength, together with small data
scattering, is of great importance for ceramic capacitors, as
reliability is still one of the biggest concerns for this type of
capacitor. Here, we demonstrated that with appropriate control of
the pyrolysis conditions, PLZT films with high reliability can be
fabricated by a PVP modified sol-gel process.
Generally, the invention provides for a process for forming a
high-quality ferroelectric PLZT films were prepared by a modified
sol-gel process. Surprising, the step-wise preheat treatment was
effective to achieve high quality PLZT films, as it reduced the
number and the size of the defects left by the decomposition of
sol-gel modifier (PVP). The PLZT films prepared by the SPT process
exhibited superior dielectric properties: dielectric constant
.apprxeq.860, dissipation factor .apprxeq.0.06, leakage current
.apprxeq.5.4.times.10.sup.-9 A/cm.sup.2, and breakdown strength
.apprxeq.2.1 MV/cm. These values are comparable to those of the
films grown by the sol-gel method without PVP addition. This
process is believed to be applicable for fabrication of
film-on-foil capacitors with thickness >1 .mu.m and preferably
for capacitors with thickness >10 .mu.m, for power
electronics.
It is to be understood that the above description is intended to be
illustrative, and not restrictive. For example, the above-described
embodiments (and/or aspects thereof) may be used in combination
with each other. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from its scope. While the dimensions
and types of materials described herein are intended to define the
parameters of the invention, they are by no means limiting, but are
instead exemplary embodiments. Many other embodiments will be
apparent to those of skill in the art upon reviewing the above
description. The scope of the invention should, therefore, be
determined with reference to the appended claims, along with the
full scope of equivalents to which such claims are entitled. In the
appended claims, the terms "including" and "in which" are used as
the plain-English equivalents of the terms "comprising" and
"wherein." Moreover, in the following claims, the terms "first,"
"second," and "third," are used merely as labels, and are not
intended to impose numerical requirements on their objects.
Further, the limitations of the following claims are not written in
means-plus-function format and are not intended to be interpreted
based on 35 U.S.C. .sctn.112, sixth paragraph, unless and until
such claim limitations expressly use the phrase "means for"
followed by a statement of function void of further structure.
The present methods can involve any or all of the steps or
conditions discussed above in various combinations, as desired.
Accordingly, it will be readily apparent to the skilled artisan
that in some of the disclosed methods certain steps can be deleted
or additional steps performed without affecting the viability of
the methods.
While the invention has been particularly shown and described with
reference to a preferred embodiment hereof, it will be understood
by those skilled in the art that several changes in form and detail
may be made without departing from the spirit and scope of the
invention.
While the invention has been particularly shown and described with
reference to a preferred embodiment hereof, it will be understood
by those skilled in the art that several changes in form and detail
may be made without departing from the spirit and scope of the
invention.
* * * * *